In the early days of space systems development, mission requirements were commonly met with one or two satellites. The best a ground-based user could hope for was an occasional pass over his geographical area of interest. The low-orbiting satellites of the time also had a fairly limited view of field of a given point on earth. The first real breakthrough was probably the use of highly elliptic communications satellites with twelve hour periods, developed by the Soviet Union in their Molniya program. Indeed, these orbits are commonly referred to as "Molniya orbits". The next breakthrough was the development and application of the geostationary (circular, equatorial, 24-hour satellites) which appear to be stationary in the sky to a ground observer, since their period of rotation matches the earth's by virtue of their placement at a precise and unique altitude. These geostationary satellites are extensively used for commercial and military communications; they are also used for weather satellites (cloud cover and weather patterns being quite clear even from the geostationary altitude of roughly 19,325 nautical miles), and in a number of other applications.
To an ever-increasing degree, space systems have become multi-satellite, coordinated orbital arrays intended to provide continuous, global service to both commercial and military users for an ever-widening field of mission applications. A typical example is the GPS/NAVSTAR position-fixing system, capable of locating platforms equipped with receiver sets to within a few dozen feet. This system operates with 21 satellites placed in a precisely-coordinated circular twelve-hour constellation or array since it is required that at least four satellites be in view of the terrestrial user with sufficient separation between satellites to facilitate the calculation of an accurate fix. Another system for satellite communications consists of four satellites in geostationary orbits around the equator. Unfortunately, due to their equatorial location, there are gaps at and near the polar regions. These regions are assuming a greater importance in the future.
Without a doubt, the most basic requirements for most satellite systems is that of geographic coverage. The term "continuous global coverage" appears with ever-increasing frequency in the development of new systems. Although a few systems are capable of a degree of "over-the-horizon" operation, the vast majority of systems require a clear line-of-sight from the satellite to the terrestrial user. This would include any optical, or high-frequency radio link type system or the like.
In the late 1960's, it was thought that six satellites were required to provide continuous global coverage. It was assumed that one ring of three circular equatorial satellites could cover all but the regions around the North and South Poles. A second ring of satellites could be placed in a highly inclined or polar orbit, but to be effective in covering both poles, it would be necessary to place at least three satellites in this inclined plane (because of equatorial crossing considerations). In 1970, an Englishman, John Walker, of the Royal Aircraft Establishment, came up with a novel five-satellite continuous coverage model. Using circular, synchronous (24-hour) orbits, two satellites were placed in geostationary locations on the equator 84.degree. apart. On the far hemisphere, three satellites were placed in highly inclined (78+ inclination) orbits, with the same longitude for ascending nodes. This resulted in a large figure-eight ground pattern, with at least one satellite always in the Northern Hemisphere, and another satellite always in the Southern Hemisphere (the worst case being when one satellite was crossing the equator.) Thus, both North and South Poles were covered by this three-satellite figure-eight.
In 1983, the present inventor designed a three satellite cubic array, or constellation, which provided complete, continuous coverage of one hemisphere (either the Northern or Southern Hemisphere). The same year, I also invented an agumented cubic array by the addition of one extra satellite in a circular equatorial orbit and having one-half the period of the other three satellites. These constellations were disclosed in a paper presented by the inventor at the August 1984 meeting of AIAA/AAS Astrodynamics Conference entitled "AIAA-84-1996. Three-and Four-Satellite Continuous-Coverage Constellations". This array provides continuous global coverage with four satellites. The satellites in these constellations do not have a common period, thus the design gain margins or resolutions of the three common-period satellites must be increased over that of the fourth (circular-orbit) satellite (which is at a lower altitude). In addition, the minimum constellation period (i.e. period of the higher satellites) for continuous global visibility is approximately 78 hours. Further, the two hemispheres are covered unevenly with greater mean coverage being provided in the selected hemisphere containing the apogees of the three longer-period satellites.
Accordingly, objects of the present invention are as follows.
Provision of continuous global coverage with minimum number of satellites.
Provision of continuous global coverage with satellites having the maximum possible value of minimum visibility or look angle.
Provision of continuous global coverage with a multi-satellite constellation or array which neither interferes with, nor receives interference from, satellites in more conventional arrays, and particularly with those satellites placed in geostationary orbits at or near the plane of the equator.
Provision of a higher than synchronous (greater than 24-hour period) multi-satellite array which is more survivable against possible hostile attack due to the fact that it is more difficult to locate, track and target by virtue of being orbited at a greater distance from earth.
Provision of a higher than synchronous (greater than 24-hour period) multi-satellite array whose orbits are perturbed to a lesser degree than are those of synchronous or lower altitudes, by the oblateness or other anomalies of the earth's surface or internal mass distribution.
Provision of a multi-satellite array designed in inertial coordinates so that any non-integral constellation period greater than the minimum critical period is usable for the specific requirements goal.
Provision of a multi-satellite array with small to moderate eccentricities which do not require excessive variations in gain margins, resolution, or detectability limits. (An example of a multi-satellite constellation having these objectionable wide variations in gain margins is the Soviet Molniya System, with an eccentricity of approximately 0.7).
Provision of a multi-satellite array, intentionally designed to be non-synchronous so as to even satellite exposure to natural or man-made interference within the array, and/or to move or rotate new, remaining array satellites into operational positions should such interference succeed in negating one of the array satellites.
Provision of a multi-satellite array, wherein each satellite serves as host platform to a variety of sensor or mission payloads, the latter then being capable of benefitting from the continuous coverage positioning of the host platform.
Provision of a multi-satellite array, wherein the inclinations of the individual satellites may exceed the maximum value of the sun's declination, thus minimizing the amount of interference by the sun on the satellite or receiving ground station. Such interference might be of two types:
a. With the satellite exactly between earth and sun, solar energy, sun spots, etc., may blank out signals from satellite to earth.
b. With the satellite in the shadow of earth, electric power shortages may occur in the satellite due to reliance on solar-cell electric battery charges.
Provision of a multi-satellite array by replication, overlay, and/or time-phasing similar or identical four-satellite arrays to give added redundancy in the numbers of satellites visible from a ground observation.
Provision of a multi-satellite array wherein the mean coverage is substantially identical for both hemispheres.
Provision of a multi-satellite array wherein all satellites have a common period, and so can be designed to operate satisfactorily with the same margins or resolutions.
Other objects and many of the attendant advantages of the present invention will become apparent by reference to the following description when considered in connection with the accompanying drawings.